This invention relates generally to seismic exploration of substrata beneath bodies of water and, more particularly, to a marine seismic cable for sensing reflected seismic waves from such substrata.
Marine seismic exploration is often conducted by towing a seismic streamer at a given depth through the ocean or other body of water. The streamer is provided with a plurality of acoustic sensitive transducers disposed at appropriate intervals along the length thereof. Acoustic wave energy is provided in the vicinity of the cable by an air gun or other suitable means; this wave energy travels downwardly through the earth with a portion of it being reflected upwardly at levels where there is a contrast in the velocity propagation characteristics of the strata. The seismic transducers sense the acoustic waves produced in the water by the upwardly traveling seismic reflections and provide electrical signals indicative thereof to suitable processing and recording equipment located on the seismic vessel that is towing the streamer.
The magnitude of the reflected signals is extremely small, thus making it essential to minimize extraneous noise. One source of such noise is the longitudinal waves which are propagated along the tension member of the seismic cable by the unsteady movement of the seismic vessel as it heaves, pitches and rolls in an unsteady sea. This unsteady motion which is transmitted to the cable via its attachment to the vessel may also result in lateral, shear, Rayleigh and torsional waves depending on the structure of the cable. These various waves are detected by the seismic transducers, thereby reducing the signal to noise ratio of the seismic signals.
Therefore, it is an object of the present invention to provide a marine seismic cable in which the seismic transducers are isolated from the waves generated in the cable by the nonuniform movement of the seismic vessel that is towing the cable.